AMS Forum: Climate Change Manifested by Changes in Weather


Research Issues on Tropical Convection and Two-way Interaction with the Large-scale Flow

Chidong Zhang, University of Miami/RSMAS, Miami, FL; and D. B. Parsons, J. Tribbia, and M. Moncrieff


The treatment of tropical convection in global models has proven to be a vexing problem for the atmospheric sciences. Current global models have difficulty in accurately representing such basic characteristics of tropical convection as the diurnal cycle, the partition between convective and stratiform rainfall, the structure of the Inter-Tropical Convergence Zone, and the location of monsoon rainfall relative to topographic features. This poor representative of tropical convection in numerical models has numerous profound implications for society. For example, on a daily basis, the inability to predict large-scale organized convective systems in the tropics literally translates to larger numbers of weather fatalities, injuries and property damage than would occur in the absence of these forecast failures. These forecast failures are not limited to just the tropics and subtropics as the distribution of organized, large-scale deep convection in the tropics influences weather in the higher latitudes through Rossby wave trains, tropical cyclones and other processes that produce high-impact weather in middle latitudes. Thus, our current weaknesses in the treatment of the tropics and tropical convection in global weather prediction systems is of such importance that it is considered to be a critical barrier to obtaining more accurate week-two weather predictions.

Tropical convection is of similar importance to climate modeling. For example, the improper treatment of convection and how it couples with the large-scale flow translates into weaknesses in our ability to simulate sub-seasonal tropical variability, including the Madden-Julian Oscillation and westerly wind bursts, which in turn make it difficult to simulate and predict the El Nino Southern Oscillation (ENSO) with coupled atmosphere-ocean models. Variability alone is not the only problem as significant biases exist in climate and weather predictions of the rainfall within the tropics even for climatological means. Given the importance of water to our exponentially growing society, these failures in the treatment of rainfall make it difficult for society to rely on climate change projections in critical areas such as planning water resources for our exponentially growing populations. On a broader scientific level, our fundamental failures in simulating tropical convection do not bode well for attempts to simulate key aspects of the Earth System ranging from atmospheric chemistry (e.g., the carbon cycle, aerosol-radiative effects) to land-surface interactions in the bio-geo sciences and the behavior of the coupled atmosphere-ocean system.

This presentation will discuss an overview of highlighting some of the problems associated with representing deep tropical convection in climate and weather prediction models. The presentation will also discuss the possible research priorities and strategies developed from a retreat on the subject of tropical convection and two-way scale interaction held at NCAR during July 2006. The retreat included representatives from the climate and weather community. The priority research goals included: i) reduction in the errors and biases currently observed in numerical simulations of convection in the tropics; ii) advancing knowledge of how tropical convection couples with the large-scale within the tropics to produce circulations such as the MJO; iii) improved representation and advancing knowledge on how tropical convection impacts the high impact weather in the middle latitudes. One proposed strategy was long duration simulations over broad areas of the tropics with resolution to resolve tropicasl convection.


Session 3, THORPEX Special Session: Impacts of Processes over the Tropical Pacific and Indian Oceans on Mid-Latitude Weather and Predictability
Wednesday, 17 January 2007, 1:30 PM-5:15 PM, 214A

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